1. Field of the Invention
The present invention is directed to a method for transforming a substrate with edge contacts into a ball grid array, a ball grid array manufactured according to this method, and flexible wiring for the transformation of a substrate with edge contacts into a ball grid array
2. Description of the Prior Art
In the manufacturing of multichip modules (MCM), i.e. systems with one or more unencapsulated integrated circuits on a flexible circuit (conductor pattern) film, the problem arises of contacting this module on an element that is a continuation of the circuit. Specially formed edge terminals are conventionally used that are then soldered. e.g. in the strap soldering method, with a circuit board that continues the circuit. As in the case of encapsulated components for surface mounting, the increasing number of terminals and the drive towards miniaturization leads to an ever-smaller terminal grid. This in turn causes difficulties in mounting, from the application of the soldering paste in a defined manner to the short-circuit-free soldering with the soldering strap.
With respect to the above-named difficulties, ball grid arrays (BGAs) are increasingly being used both for individual chips and for multichip modules, in which BGAs the terminals are distributed in planar fashion and thus in an essentially coarser grid than in the conventional forms on the underside of the housing. Each of these planarly distributed terminals supports one or more humps that can be melted, known as balls, which provide the connection with the conductors (circuit) in a melting process (cf. DE-Z Productronic Vol. 5, 1994, pages 54, 55).
If the wirings of the multichip modules are manufactured in circuit board technology (known as MCM-L), the ball grid array can be realized easily. Through-contacts through the substrate enable a planar arrangement of the terminals or of the meltable humps on the underside. Likewise, given multilayer ceramics manufactured in sinter technology with a thick layer wiring (called MCM-C), a planar arrangement of the terminals or of the meltable humps is possible.
If, however, the multichip modules are manufactured by deposition of layers on an unstructured substrate, e.g. ceramic (called MCM-D), this substrate will only outwardly have edge contacts for the connection; i.e., assembly as a ball grid array is not possible without taking further steps.
From PCT Application WO 89/10005, a chip packaging is known in which a preformed substrate has a receptacle at its upper side for receiving an integrated circuit, and has four ribs on its lower side that project beyond the outer edge. A flexible wiring, with an opening corresponding to the receptacle, is applied on the upper side of the substrate, and is bent downwardly around the ribs such that the ends of the individual conductors form terminals in the region of the ribs. These terminals, however, are edge contacts located underneath; i.e., a realization as a ball grid array is also not possible in this technique.